The invention relates to a flotation cell and a method for improving flotation effect in the flotation cell by feeding flowing material for slurry dilution to the top region of the cell below the froth zone.
In flotation machines, the desired valuable mineral particles are put to contact with air bubbles by means of chemicals. The air bubbles rise on the surface of the slurry layer and form a foam layer, the height whereof in free space is normally only 5% of the height between the bottom of the flotation machine and the foam outlet froth lip.
The U.S. Pat. No. 5,039,400 relates to a flotation machine where the slurry and froth space is provided with at least one downwards narrowing member, whereby the froth volume and the froth surface area can be regulated in order to form a thick froth bed. The height of this froth bed is between 20-40% of the total height of the flotation cell. Inside the froth bed, there is further arranged a washing system for cleaning the concentrate. Owing to the downward washing effect by the process liquid, the remaining small slime-forming particles both entrained between air bubbles and on the surfaces of the froth bubbles can be removed. By means of washing liquid, the slime-forming fine particles are settled to the slurry tank of the flotation cell and are advantageously discharged through the slurry outlet provided in the bottom part of the cell.
In this U.S. Pat. No. 5,039,400 the washing liquid, normally water, can be used also as dilution water for the slurry to be removed from the flotation machine via the bottom outlet. However, because washing liquid is fed into the froth bed, washing liquid will decrease the effect of the froth bed and thus the supply for the concentrate will be diminished, i.e. the recovery of the valuable mineral particles will be lower while the concentrate grade will increase.
The U.S. Pat. No. 5,923,012 describes a flotation method and apparatus for treatment of cyclone sands, where a flash flotation machine is provided with a top outlet for progressive removal of the surface froth from the upper zone via a launder to provide flotation concentrate, a bottom outlet for progressive withdrawal of the relatively dense component of the slurry from the lower zone and a side outlet for proggressive removal of the relatively less dense component of the slurry from the intermediate zone in the tank.
The dilution water in the U.S. Pat. No. 5,923,012 is fed to the flotation machine through the feed chute where from also the slurry to be concentrated is fed. The dilution water is then dispersed throughout the flotation machine. However, the main effect is directed to the pumping flows of the rotor in the lower part of the flotation machine. Thus for instance the intermediate zone of the flotation zone is not very effective as a target for the dilution water and, therefore, the slurry density in the slurry removed from the flotation machine can essentially be changed and is not advantageously maintained for further downstream treatment.
The object of the present invention is to eliminate at least some drawbacks of the prior art and to achieve a flotation machine and a method for improving flotation effect where flowing material for slurry dilution, as dilution water, is fed into a flotation machine below the froth bed, but essentially above the top part of the rotor of the flotation machine positioned in the lower part of the flotation machine. The essential features of the invention is enlisted in the enclosed claims.
In accordance with the invention, the flotation machine has members for feeding material to be treated in the flotation machine, members for removing the treated material out of the flotation machine at least in the upper part of the machine and in the bottom part of the flotation machine and a mixing mechanism comprising a stator and a rotor located inside the cell and beneath the feeding of the material. In the flotation machine there are also air supply means for supplying air to the mixing mechanism and forming a froth bed in the upper part of the flotation machine as well as members for adjusting the free space of the froth bed created in the flotation machine. In the upper part of the flotation machine for adjusting the slurry and the froth space there is at least one downwards narrowing, advantageously conical or wedge-shaped member. In accordance with the invention dilution water is fed below the froth zone, but essentially above the top part of the rotor of the flotation machine, by installing at least one conducting member for the dilution water inside the flotation machine.
The flowing material for slurry dilution in accordance with the invention can be liquid, as water, or diluted slurry in which the solids content is advantageously smaller than in the slurry already in the flotation machine. Further, the flowing material for slurry dilution can also be an overflow received for instance from a thickener.
The conducting member for the dilution member is for instance a pipe which is installed inside the tank of the flotation machine so that the conducting member is supported by the wall of the tank or by the adjusting member for the slurry and the froth space. It is also possible to install the conducting member so that the conducting member is supported by the stator of the flotation machine or by any other suitable surface inside the tank of flotation machine. The conducting member can also be installed in the froth zone. Regardless of the supporting surface the conducting member is installed so that the flowing material for slurry dilution, as dilution water, is directed into the slurry essentially above the top part of the rotor of the flotation machine, but below the froth zone.
In one embodiment for the dilution water to be fed into the flotation machine in accordance with the invention at least one adjusting member for the slurry and the froth space is provided with means to direct the dilution water to the area beneath the froth bed. The means is advantageously at least one conducting member which is installed to the outer surface of the adjusting member and the dilution water flows through the conducting member and is directed into the slurry. The dilution water can also be fed to the flotation machine by at least one conducting member which is installed inside the adjusting member. An opening is formed in the adjusting member in order to allow only the end part of the conducting member to be placed outside of the adjusting member. In accordance with one embodiment of the invention the conical part of the adjusting member operates as a vessel where from the dilution water is fed to the slurry positioned in the end part of the cone placed inside the slurry. Advantageously in any embodiments of the invention at least one nozzle is installed in that end of the conducting member which end is immersed in the slurry.
When feeding the dilution water in accordance with the invention to the top of the rotor the dilution water is sucked out of the conducting member by rotor flows which are directed to the top part of the slurry area of the flotation machine. Because the dilution water is fed essentially directly to the area where the dilution water has the advantageous effect for further treatment of the material to be removed from the flotation machine, the amount of the dilution water is also diminished.
The diminished amount of the dilution water makes it possible to obtain a higher bottom outlet density for the non-flotatable material. It also allows the froth concentrate being removed from the upper zone to contain a higher proportion of valuable mineral. Because the dilution water is under the effect of the secondary rotor flows, the effect of the dilution water is diminished in the lower part of the flotation machine in the area of the primary rotor flows and thus the density for the non-flotatable material in the bottom outlet is still high and advantageous for further treatment. The result therefrom is a low density area in the top part, beneath the froth bed of the flotation machine.